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Abstract

The investigation of the Martian atmosphere is of key importance for an understanding of the planets present and past. Passive limb observations of thermal radiation at submillimeter wavelengths in the 320–350-GHz range by use of a state-of-the-art satellite receiver on a low Mars orbit allow important parameters such as the mixing ratios of H2O, HDO, 12CO, 13CO, O3, and H2O2 as well as the thermal profile to be retrieved with high precision and unprecedented vertical range and resolution, providing valuable information for better understanding of the planet’s water cycle, atmospheric dynamics, and photochemistry. The feasibility of these kinds of measurement is demonstrated by means of model simulations based on realistic atmospheric, spectroscopic, and instrumental parameters. Temperature can be retrieved to ∼90 km with half-scale height vertical resolution from single-scan measurements of emission lines of the long-lived species 12CO and 13CO. The global water-vapor distribution can be measured even under dry or wet conditions with good vertical resolution from the surface to ∼45 km, and simultaneous observations of HDO allow useful information on the D/H ratio up to an altitude of ∼30 km to be derived. The sensitivity of the limb-sounding technique also permits information on the photochemically important minor species O3 and H2O2 to be obtained. It is shown that spectral averaging may improve precision, altitude range, and resolution of the retrieved profiles. Other frequency bands are explored, and the 435–465-GHz range is suggested as a possible alternative to the 320–350-GHz range.

References

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Table 2

Theoretical Capabilities of a Submillimeter-Wave Limb Sounder in the 320–350-GHz Range for Observation of Key Parameters in the Martian Atmospherea

Band

Species (Integration Time, Retrieval Grid Step)

ν0(GHz)

Altitude Resolution(km)

Altitude Range(km)

1-σ Precision(ppv or K)

A

12CO(1 s)

345.796

≤5 (<40 km)

0–100

4–10 × 10−5 [5–10%]

13CO(1 s)

330.588

≤7 (<40 km)

0–75

5–7 × 10−7 [5–10%]

T (1 s)

4–5 (<55 km)

0–85

3–6 K

T (25 s)

4–5 (<55 km)

0–110

1–2 K

T (1s, 10 km)

10 (<80 km)

0–100

≤1 K (<50 km)

T (25 s, 10 km)

10 (<100 km)

0–120

≤1 K (<70 km)

T (5 s, nadir)

6–16

0–70

∼5 K

B

H2O

325.153

∼4 (<70 km)

0–100

≤1 × 10−5 (<50 km)

H2O (25 s)

∼4 (<85 km)

0–110

≤ 3 × 10−6 (<50 km)

H2O (1 s, 10 km)

10

0–100

≤2 × 10−6 (<60 km)

H2O (25 s, 10 km)

10

0–110

≤1 × 10−6 (<75 km)

H2O (5 s, nadir)

10–20

4–60

4×6 × 10−5

C

HDO (1 s)

335.396

∼5 (<35 km)

0–60

10–15 × 10−8 (<40 km)

HDO (25 s)

∼4 (<35 km)

0–75

8–15 × 10−8 (<40 km)

HDO (1s, 10 km)

10

0–60

2–3 × 10−8 (<40 km)

HDO (25 s, 10 km)

10

0–70

3–6 × 10−9 (<40 km)

D

O3 (1 s, 10 km)

326.901

10

0–70

1–3 × 10−8 (<50 km)

O3 (25 s, 10 km)

10

0–80

2–6 × 10−9 (<50 km)

H2O2 (1 s, 10 km)

326.982

10

0–60

≤1 × 10−8 (<25 km)

H2O2 (25 s, 10 km)

10

0–70

≤ 2 × 10−9 (<25 km)

a Shown are target line frequencies, altitude resolutions derived from the averaging kernel functions (FWHM), altitude ranges where the a priori contribution is negligible, and single-scan precision for 4- and 10–km step retrieval grids. Results obtained for 5-s nadir observations as well as for 25-s limb (zonal mean) retrievals are also provided for comparison.

Tables (2)

Table 1

Target Lines and Back-End Spectrometer Configuration for the 320–350-GHz Range

Table 2

Theoretical Capabilities of a Submillimeter-Wave Limb Sounder in the 320–350-GHz Range for Observation of Key Parameters in the Martian Atmospherea

Band

Species (Integration Time, Retrieval Grid Step)

ν0(GHz)

Altitude Resolution(km)

Altitude Range(km)

1-σ Precision(ppv or K)

A

12CO(1 s)

345.796

≤5 (<40 km)

0–100

4–10 × 10−5 [5–10%]

13CO(1 s)

330.588

≤7 (<40 km)

0–75

5–7 × 10−7 [5–10%]

T (1 s)

4–5 (<55 km)

0–85

3–6 K

T (25 s)

4–5 (<55 km)

0–110

1–2 K

T (1s, 10 km)

10 (<80 km)

0–100

≤1 K (<50 km)

T (25 s, 10 km)

10 (<100 km)

0–120

≤1 K (<70 km)

T (5 s, nadir)

6–16

0–70

∼5 K

B

H2O

325.153

∼4 (<70 km)

0–100

≤1 × 10−5 (<50 km)

H2O (25 s)

∼4 (<85 km)

0–110

≤ 3 × 10−6 (<50 km)

H2O (1 s, 10 km)

10

0–100

≤2 × 10−6 (<60 km)

H2O (25 s, 10 km)

10

0–110

≤1 × 10−6 (<75 km)

H2O (5 s, nadir)

10–20

4–60

4×6 × 10−5

C

HDO (1 s)

335.396

∼5 (<35 km)

0–60

10–15 × 10−8 (<40 km)

HDO (25 s)

∼4 (<35 km)

0–75

8–15 × 10−8 (<40 km)

HDO (1s, 10 km)

10

0–60

2–3 × 10−8 (<40 km)

HDO (25 s, 10 km)

10

0–70

3–6 × 10−9 (<40 km)

D

O3 (1 s, 10 km)

326.901

10

0–70

1–3 × 10−8 (<50 km)

O3 (25 s, 10 km)

10

0–80

2–6 × 10−9 (<50 km)

H2O2 (1 s, 10 km)

326.982

10

0–60

≤1 × 10−8 (<25 km)

H2O2 (25 s, 10 km)

10

0–70

≤ 2 × 10−9 (<25 km)

a Shown are target line frequencies, altitude resolutions derived from the averaging kernel functions (FWHM), altitude ranges where the a priori contribution is negligible, and single-scan precision for 4- and 10–km step retrieval grids. Results obtained for 5-s nadir observations as well as for 25-s limb (zonal mean) retrievals are also provided for comparison.